Electrical interconnections are required for electrically connecting an integrated circuit chip to an electrical circuit and a supporting substrate. Typically, the integrated circuit has small solder balls or "bumps" attached to those regions where electrical contact will be made. The integrated circuit is in position such that the bumps contact the electrical circuit on a supporting substrate in the appropriate regions.
In addition to the conventional thick and thin film technologies wherein an electrical circuit is printed or deposited on the surface of a substrate so as to form a patterned circuitry, many methods are known for forming the electrical interconnections between an integrated circuit and the electrical circuit on a supporting substrate. Regardless of the particular soldering method used for attaching the chip to a supporting substrate, for enhanced reliability it is desirable to control the amount of solderable surface area on each of the integrated circuit contacts on the substrate. If the solderable surface area of the contact, as defined by the conductor area enclosed by a surrounding dielectric, is too large, the mass of solder which forms the bump has a propensity to shear off during exposure to the widely varying temperature ranges of operation. Therefore, the actual contact area between the individual solder bump on the integrated circuit and the contact region of the conductor must be optimally minimized.
Generally, the amount of contact area should be controlled and minimized by providing a conductor having a predetermined, optimal width in the contact region. Further, a dielectric region should be provided, referred to as a solder dam or solder stop, on the conductor in the contact area to prevent the solder from wicking or flowing down the conductor.
Flexible solder bumps serve a variety of functions. Generally, flexible solder bumps serve: as an electric connection between the integrated circuit chip and the conductors on the circuit board; to keep the integrated circuit chip elevated above the board so that the solder flux residue may be washed off of the circuit board and away from the integrated circuit chip where the flux can cause damage; to dissipate heat away from the integrated circuit chip; and to accommodate differences in thermal expansion between the integrated circuit chip and the circuit board during operation of the device. Solder bumps when uniform in dimension allow for an overall higher current density through the device. The flexible solder bumps may be attached to the chip by a variety of methods including plating or by using solder balls.
Prior to the present invention, solder dams or solder stops consisted of thick film of dielectric films deposited over the conductor layer, wherein a trough is patterned within the dielectric layers. Within the trough there is no dielectric material present. The solder is retained in the trough and between the stops to connect the integrated circuit and the conductors.
Numerous disadvantages exist with respect to prior art solder dam or stop designs. The alignment between the conductor layer and the solder dam layer must be maintained quite strictly or substantial misalignment results between the two layers. When there is even slight misalignment between layers, the conductors do not align with the solder dam or conductive bumps on the integrated circuit. Therefore, there may be electrical connection which are formed only partially or not at all, resulting in diminished integrity of the electrical connections and the electrical circuit as a whole. Further, such designs require multiple printing and firing steps. First, the conductive lead must be printed and then fired. Second, the solder dam must be printed over the fired conductive lead and the device fired again. Still further, the use of a second printed layer to provide a solder dam or stop cannot be printed with great accuracy and often have widths arranging from 0.010" to about 0.020".
It would therefore be desirable to provide an electrical interconnection lead prepared from a single printing and firing, and which provided a solder stop with great precision and minimal width.